1. Field of the Invention
The present invention generally relates to printing papers or sheets suitable for use in thermal transfer systems, especially sublimation type thermal transfer systems and manufacturing methods therefor. More particularly, the present invention is directed to a printing paper or sheet which has excellent anti-sebaceous characteristic as well as excellent writing characteristic simultaneously, and a manufacturing method therefor.
2. Description of the Related Art
There is widely used a thermal transfer recording system in which an ink ribbon is heated through a thermal head or by laser or the like in accordance with image information to transfer a dye from the ink ribbon onto a printing sheet through thermal melting, thermal diffusion or sublimation to thereby form an image on the printing sheet. Particularly, in recent years, a sublimation type thermal transfer recording system receives a considerable attention, in which a thermal dispersion dye such as a sublimation dye or the like is used to form a full color image having continuous gradations. For example, an attempt to heat an ink ribbon selectively in accordance with an image signal of a video image to thereby form an image on a video printing sheet has been made.
As such a video printing sheet used is that which is obtained by forming a dye receiving layer on a sheet-like support made of polypropylene or the like. The dye receiving layer is a layer which receives a dye transferred thereto from the ink ribbon by heating and preserves an image formed from the dye. Examples of a resin heretofore used for constituting the aforementioned dye receiving layer include materials which can take dye well, for example, polyester, polycarbonate, polyvinyl chloride, vinyl chloride copolymer such as vinyl chloride-vinyl acetate copolymer, and thermoplastic resin such as polyurethane resin, polystyrene, AS resin, ABS resin, etc.
Further, in recent years, various attempts to improve sensitivity to make the formation of a sharp image possible and improve the weather resistance, light resistance and thermal stability of the resulting image to preserve the image stably have been made on the resin for constituting the dye receiving layer. For example, in order to improve the light resistance and weather resistance of the resulting image, use of cellulose ester mainly as the resin for constituting the dye receiving layer has been proposed (U.S. Pat. No. 5,278,130).
With the spread of thermal transfer recording system, not only the sensitivity, and the weather resistance, light resistance and thermal stability of the resulting image are required to be improved but also the following characteristics are required to be satisfied. That is, it is required to give a printing sheet such excellent anti-sebaceous characteristic that deposition of a dye on a human hand is prevented when the dye receiving layer having an image formed is rubbed with the hand, and it is required to give a printing sheet such excellent writing characteristic that characters written on the dye receiving layer with oil ink writing goods exhibit excellent fixing properties.
The aforementioned conventional printing sheet, however, has a problem in that anti-sebaceous characteristic and writing characteristic cannot be satisfied simultaneously. That is, in order to improve anti-sebaceous characteristic to prevent the deposition of a dye onto a human hand rubbing an image surface, it is required that a resin having high volatility to prevent the penetration of sebaceous oil into the dye receiving layer is used as the resin for constituting the dye receiving layer. On the contrary to the case of improvement of anti-sebaceous characteristic, in order to improve writing characteristic of the printing sheet to make it possible to write characteristics or the like on the printing sheet directly with an oil pen, it is required that a resin which has not high volatility but high lipophilic property so that a dye or dye-dispersed ink can penetrate into the dye receiving layer sufficiently is used as the resin for constituting the dye receiving layer. As described above, with respect to the resin for constituting the dye receiving layer, the property required in order to improve anti-sebaceous characteristic and the property required in order to improve writing characteristic are antinomic. It is therefore difficult to improve the two characteristics simultaneously. When, for example, an isocyanate cross-linking agent is contained in the dye receiving layer to perform a cross-linking reaction for the purpose of improving anti-sebaceous characteristic, oil ink can hardly penetrate into the dye receiving layer, resulting in lowering of writing characteristic.